If only all writers would get to the action--and general conflict--of a story as quickly as the master, William Shakespeare, did in his everlastingly popular Romeo and Juliet. One reason for the high energy scene is the same one that all writers and filmmakers should hope to achieve: to immediately draw reader or viewer interest. What better way to please an audience than with a sword fight in the very first scene? In this case, it could also relate to the exhuberance of youth theme that runs through the play as well as to quickly establish the theme of recurring violence. Another reason would be to emulate the tumultuous times of the city, Verona, due to the bad blood between the warring families. Additionally,

Shakespeare intended to begin his play with a street fight in order to appeal to the common people and immediately gain the attention of the groundlings who might become restless quickly.

I agree that this would be a very quick and easy way to grab the attention of all viewers, and directly entertain most. However, I also think he did this stylistically, because if you think about it, the same amount of violence is almost cyclical within the text. Also, it helps the viewer/reader understand the family conflict and history of the feud.